Cedar: JSON template is in scope missing slot-id validation allows link panic and decision divergence
Published Sources: OSV
What is affected, what to do
The available sources name no fixed version. Check the vendor guidance in the references below.
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Read in glossary- 30.00 Methodology
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Read in glossary- no public PoC known
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Classification under NIS2 and CRA
Rule-based classification from exploitation status and severity. It does not replace a case-by-case assessment.
NIS2
NIS2 is EU Directive (EU) 2022/2555 to raise the level of cybersecurity across the Union. It substantially broadens obligations compared with the first NIS directive: more sectors, mandatory risk management, staged reporting duties and personal accountability of senior management.
Read in glossary- Handle the vulnerability in regular vulnerability handling under Art. 21(2)(e) NIS2.
CRA
The CRA is EU Regulation (EU) 2024/2847 setting horizontal cybersecurity requirements for „products with digital elements“ — from industrial controllers and software to connected consumer devices. For the first time manufacturers must ensure security across the entire product lifecycle, handle vulnerabilities and provide security updates.
Read in glossary- From 11.12.2027, Annex I Part II CRA requires manufacturers of affected products with digital elements to handle vulnerabilities without delay and provide security updates.
ISO/IEC 27001
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Read in glossary- Control A.8.8, management of technical vulnerabilities: assess relevance to your systems, plan remediation and keep evidence.
Description
Cedar users must be using all of the following features to be affected by this advisory: JSON format policies, policy templates, and the is in scope condition. This advisory only affects JSON-formatted Cedar policy templates containing is in scope conditions that are malformed according to the Cedar syntax. It does not affect syntactically well-formed policies.
Users of Cedar who author policies in the JSON format were able to write Cedar policies using the is in condition with an incorrect slot identifier in the principal and resource scope constraints (e.g., ?principal in the resource scope constraint) even though this is not allowed when parsing policies from the Cedar policy syntax. Internally, the slot identifiers were silently corrected (e.g. ?resource is placed in the resource scope constraint).
If the policy contains exactly one slot in either the principal or resource position, then any attempt to create a linked policy from the template results in an error: the internal representation of the template does not contain the slot identifier as written in the malformed JSON policy (it has been corrected silently), so attempting to fill that slot is an error. If instead the template is linked using slot identifier expected internally (i.e., not what is written in the JSON template), the linking will initially succeed, but a subsequent invariant violation will cause a panic.
When the template contains slots in both the principal and resource position, then both slots will be present in the internal representation: linking succeeds, but with each slot bound to the value intended for the other (the silent correction). The linked policy is then semantically different from the malformed policy the user intended, and any subsequent authorization decision made against that policy may lead to accesses being unintentionally allowed or denied.
Concretely, consider the following JSON format policy template intended to deny users in a particular project from accessing documents with a particular classification.
{
"effect": "forbid",
"principal": {
"op": "is",
"entity_type": "User",
"in": { "slot": "?resource" }
},
"action": { "op": "All" },
"resource": {
"op": "is",
"entity_type": "Doc",
"in": { "slot": "?principal" }
},
"conditions": []
}
Linked with ?principal = Classification::"topsecret" and ?resource = Project::"alpha", the author expects to create the policy:
forbid(principal is User in Project::"alpha", action, resource is Doc in Classification::"topsecret");
But because Cedar internally links slots by scope position (not by name), the actual policy evaluated by the engine is:
forbid(principal is User in Classification::"topsecret", action, resource is Doc in Project::"alpha");
If a user is in Project::"alpha" but not Classification::"topsecret", the forbid policy does not apply, and access is unintentionally allowed.
Patches
This advisory is patched in version 4.11.1 and 3.4.3, both available on Crates.io and NPM.
Workarounds
If you cannot update to a patched version, you can determine if you are affected by checking your JSON format policy templates for the following two patterns.
"principal": {
"op": "is",
"entity_type": "User",
"in": { "slot": "?resource" }
}
and
"resource": {
"op": "is",
"entity_type": "Doc",
"in": { "slot": "?principal" }
}